Glass ceramic and process therefor

ABSTRACT

THE INVENTION IS DIRECTED TO GLASS-CERAMIC ARTICLES HAVING A GLASS-CERAMIC MAIN BODY PORTION AND AT LEAST ONE INTEGRAL SURFACE PORTION WHICH DIFFERS FROM THE BODY PORTION. THE SURFACE PORTION IS EITHER A GLASS OR A GLASS CERAMIC HAVING A LOWER COEFFICIENT OF LINEAR EXPANSION THAN THE BODY PORTION. PROCESSES FOR MANUFACTURING THE GLASSCERAMIC ARTICLES ARE ALSO DISCLOSED.

United States l atent G F US. 0. 161-1 9 Claims ABSTRACT OF THEDISCLOSURE The invention is directed to glass-ceramic articles having aglass-ceramic main body portion and at least one integral surfaceportion which differs from the body portion. The surface portion iseither a glass or a glass ceramic having a lower coefficient of linearexpansion than the body portion. Processes for manufacturing theglassceramic articles are also disclosed.

The present application is a division of application Ser. No. 558,238,filed June 17, 1966, now abandoned.

More particularly, the present invention relates to a glass-ceramicarticle having a main body portion and at least one integral surfacelayer that differs from the body portion in some manner. The main bodyportion of the article is a glass-ceramic including at least 1% byWeight, expressed as soda mole equivalent, of an oxide of at least onealkali metal, i.e., including an alkali metal oxide in a concentrationthat, if not soda, its replacement by an equivalent molar amount of sodawould provide a composition containing at least 1% by weight of soda,based only on the substituted soda. In other words, any soda alreadypresent in the composition is not considered as being soda in thecomputation of the 1% limitation based on the alkali metal molarequivalent substitution. At least 2% by weight is preferred.

That different surface layer of the article is either a glass or aglass-ceramic that (1) has an overall composition in which (a) thelithia content when the layer is a glass-ceramic is at least equal toand when the layer is a glass is greater than the lithia content in theoverall composition of the main body portion and (b) the content ofoxide of alkali metal, having an ionic radius larger than lithium, whenthe layer is a glass-ceramic is at a maximum equal to and when the layeris a glass is less than the content of that oxide in the overallcomposition of the glass-ceramic main body portion, and (2) has a lowercoefficient of linear expansion than that of the main body portion. Thearticle has a compressive stress in that surface layer and a tensilestress in the main body portion.

The surface layer has a composition in which the mole percentageconcentration of any alkaline earth metal oxide is usually equal to themole percentage concentration of the same alkaline earth metal oxide, ifpresent, in the main body portion of the article. Some loss of alkalineearth metal oxide in the surface layer may occur during the making ofthe article in the event a high temperature is used in some cases ofheat treatment, but its content remains substantially unchanged.

In accordance with this invention, the article of the invention can bemade b carrying out one of the processes of the invention. Some of theprocesses are alternatives that can use an article of the same initialglass, whereas the use of these alternative processes or other processesis dependent upon the nature of the initial glass of the article to betreated for the manufacture of the article of the present invention.

In the article of the invention in which both the main body portion andthat surface layer are glass-ceramics, either the crystalline phase ofthe surface layer differs 3,764,444 Patented Oct. 9, 1973 from that ofthe main body portion or the glass matrix of the surface layer differsin lithia content from that of the main body portion. Both differencesmay be present. When the dilference is the lithia content of the glassmatrix, the predominant crystal in the crystalline phase of the surfacelayer may have either the same as or a greater lithia content than thatof the predominant crystal of the crystalline phase of the main bodyportion.

In carrying out all embodiments, except one, of the process of thepresent invention there is provided at some stage of the process anion-exchange treatment in which the lithia content of the surface layeris increased without a comparable increase of the lithia content of themain body portion of the initial or intermediate article. In theion-exchange step at least part of the content of an alkali metal ionhaving a larger ionic radius is replaced in the surface layer by lithiumions to obtain the product of this invention. Usually the processrequires steps after the ion exchange but in one embodiment it is thelast important step. The alkali metal ion having the larger ionic radiusthat is replaced by lithium ion is preferably sodium ion.

An illustrative embodiment of the product of the invention has as theglass-ceramic of the main body portion of the article a crystallinephase in which beta-spodumene predominates and as the surface layer aglass-ceramic in which (1) the same crystal predominates in thecrystalline phase but the glass matrix of the surface layer has a higherlithia content or (2) the surface layer has betaeucryptite crystalspredominant in the crystalline phase regardless of any difference in theglass matrix. In either event the surface layer of the article has alower average coefficient of linear expansion than that of the main bodyportion of the article and as a result the surface layer has acompressive stress and the main body portion has a tensile stress toprovide a greater fiexural strength than that of an article entirely ofthe same glass-ceramic as that main body portion.

In another illustrative embodiment, the main body portion is aglass-ceramic, that has an average coefficient of linear expansionsubstantially higher than glass-ceramic in which beta-spodumene is thepredominant crystal in the crystalline phase, and the surface layer is(1) a glassceramic with the same type of crystalline phase as the mainbody portion but with a greater concentration of crystalline phase orwith a glass matrix that has a lower linear coefficient of expansionthan the glass matrix of the main body portion, or (2) a glass withprimarily a higher lithia content than the glass-ceramic of the mainbody portion. When the surface layer is a glass, the glassceramic of themain body may have an average linear coefficinet of expansion somewhatless than, but preferably greater than, that of a thermallycrystallizable glass of the same overall composition so that due to thehigher lithia content of the glass layer in that article of theinvention the glass has an average coefficient of linear expansionsubstantially less than that of the glass-ceramic main body portion.

In one embodiment of the process of the invention, the composition ofthe thermally crystallizable glass, used to form the initial article,contains a concentration of an oxide of an alkali metal that inhibitsthe rate of bulk crystallization so that for a particular heat treatmentthe rate of such crystallization is substantially less than that of acomposition differing only by the absence of that alkali metal oxide.This inhibition can be enhanced by using a composition in which aningredient that combines with alumina and silica during bulk in situcrystallization is present in the low portion of its required range ofconcentration in the glass for the latter to be a thermallycrystalh'zable glass. The nucleant content in the glass is within therange required for bulk crystallization and is preferably above theminimum portion of the range. As a result, the glass article can beheated to an elevated temperature, e.g., 600 C. to 700 C., which issubstantially below its softening point, to obtain surfacecrystallization which provides a surface layer that does not soften whenthe article is then heat treated at higher temperatures for nucleationand finally still higher temperatures for bulk crystallization of themain body portion of the article. Of course, the temperature used forsurface crystallization, which does not require nucleation of thenucleaut in the surface layer, may be at the temperature that providessuch nucleation in the main body portion. In any event, the surfacelayer of glass-ceramic obtained by surface crystallization minimizessagging or other shape change during the bulk crystallization.

This latter embodiment of the process of the invention is useful in theother embodiments in which the article of glass-ceramic obtained by thebulk crystallization is further treated by additional steps incluing ionexchange, but is also useful in making that glass-ceramic articleitself. To accelerate the rate of surface crystallization, the originalglass article may be ion exchanged when another alkali metal oxide isone of the ingredients that participates to form a crystalline phase. Insuch case, an ion exchange step is performed on the glass to replace inits surface layer at least part of the inhibiting alkali metal ion bythe participating alkali metal ion. For example, in a glass that form bycrystallization glass-ceramic in which the crystalline phase containsbeta-eucryptite or beta-spodumene or both, sodium is the inhibiting ionand is replaced by lithium in a surface layer prior to the surfacecrystallization. In the event the foregoing process is followed byconversion of the surface layer to glass With subsequent crystallizationof the surface layer, ion exchange of the glass surface layer is notperformed prior to the crystallization of it to a glass-ceramic having adifferent crystalline phase than that of the main body portion, unless athicker surface layer is desired.

BACKGROUND OF THE INVENTION Glasses that are controllably crystallizableby a heat treatment are commonly referred to as thermally crystallizableglass compositions. The glass-ceramics are the products obtained fromthese controllably crystallizable inorganic glasses by a suitable heattreatment, and glassceramics are also referred to as thermallycrystallized glasses. Thus the term noncrystalline glass excludesglass-ceramics but for convenience the term glass is used in thisapplication to provide such exclusion and, therefore, to meannoncrystalline glass.

There are many types of silicate glasses that are thermallycrystallizable glass compositions. A glass-ceramic body contains manysmall crystals in a glass matrix. The crystalline phase ofglass-ceramics can contain one or more crystalline materials. Thecrystalline materials that are formed depend upon the originalcomposition of the thermally crystallizable glass and often depend uponthe nature of the heat treatment.

The expansion coeflicient of thermally crystallizable glass is dependentupon the glass composition. There can be a substantial differencebetween the expansion coefficients of thermally crystallizable glassesthat are not members of the same glass system. Also, the expansioncoefiicients of the glass-ceramics can differ greatly. The actualexpansion coefiicient of a glass-ceramic depends on the compositionalingredients and on the temperatures and times of the heat treatment usedto form the glassceramic from the thermally crystallizable glass.

Articles of glass-ceramic material are made by melting batch ingredientsto provide molten thermally crystallizable glass and thereafter formingfrom the molten glass by conventional means, such as press molding,casting, blow molding, and tube and rod drawing, useful glass articles.One type of useful article is tableware such as plates, cups, and teapots. Tableware is usually made by pressing in a mold or by blow moldingtechniques. The articles of thermally crystallizable glass are subjectedto a controlled heat treatment to convert the glass to a glass-ceramic.

Some glass-ceramics are compositions that contain one or more alkalimetals, expressed as oxide as part of an overall composition alsoexpressed primarily as oxides. Many of the thermally crystallizableglass compositions are of the lithia-alumina-silica system containing aminor amount of at least one nucleating agent for the glass, such as ZrOTiO and SnO By controlled in situ crystallizathere is obtainedglass-ceramic that contains in a glass matrix predominantlylithia-containing crystalline phases, either beta-eucryptite orbeta-eucryptite-like crystals or beta-spodumene or beta-spodumene-likecrystals, or both, as indicated by X-ray diffraction data.

Copending US. patent application Ser. No. 352,958, filed on Mar. 18,1964 now US. Pat. 3,380,818, by William E. Smith and entitled Glasses,Ceramics and Methods, with a common assignee, discloses and claimsanother class of glass and glass-ceramics that comprise silica, alumina,lithia, magnesia and a limited amount of both zirconia and titania. Theapplication is hereby incorporated by reference.

U.S. application Ser. No. 352,958 describes the manner of heat treatmentto convert the crystallizable glass composition to glass-ceramic. Themaximum temperature reached in the heat treatment ranges from about 1400F. to 2100 F. and the period of time at the final temperature used isdependent upon the degree of crystallization desired in the product andupon the actual maximum temperature. When the maximum temperature islimited to the range of about 1400 F. to about 1675 F. it is indicatedthat the patent application that in the crystalline phase that is formedbeta-eucryptite or beta-eucryptitelike crystals predominate. When thefinal or maximum temperature of the heat treatment is above about 1650F. the crystals formed constitute a mixture of beta-encryptite andbeta-eucryptite-like crystals and beta-spodumene and beta-spodumene-likecrystals. At the maximum heat treatment temperatures of about 1800 F. to2100" F. the crystalline phase is primarily beta-spodumene crystals andbeta-spodumene-like crystals.

The glass-ceramic of that patent application has an expansioncoefficient that is dependent upon the final temperature of the heattreatment. When the glass-ceramic results from a final heat treatmenttemperature of a maximum of about 1675 F., the expansion coefficient issubstantially lower than when a higher final temperature for the heattreatment is used. In other words, the glassceramic, in which thecrystalline phase can be considered as being beta-eucryptite, has asubstantially lower expansion coefiicient than the glass-ceramic of thesame composition having a crystalline phase that can be considered asbeing beta-spoclumene.

Copending US. patent application Ser. No. 464,147 filed by Clarence L.Babcock, Robert A. Busdiecker and Erwin C. Hagedorn on June 15, 1965 andnow abandoned, and entitled Product and Process for Forming Same withcommon assignee discloses and claims a further class of thermallycrystallizable glasscompositions and glassceramics made from theseglasses. That application is hereby incorporated by reference.

The Babcock et a1. patent application Ser. No. 464,147 discloses thediscovery that a crystallizable glass composition, containing thefollowing essential components, present in the glass composition in thefollowing Weight percent limits, can be treated at a finishingtemperature that can be varied at the maximum within about 50 to F. ormore, without affecting the substantially uniform, low expansioncharacteristics imparted to the transparent glass-ceramic which isformed, the glass-ceramic having a coefiicient of linear expansion ofabout 10 10- to about 10 10-' per C. over the range 0 to 300 C.

where the ratio of (CaO+MgO+ZnO+Na O to Li O is less than 2.4 and theratio of SiO to A1 is no more than 3.8 and is usually no more than 3.3.An example of this class of thermally crystallizable glasses thatprovides a glass-ceramic having an average coefficient of linearexpansion of 0+10- C. (0-300 C.) has the following theoreticalcomposition and for an actual tank batch had the following analyzedcomposition, expressed as various oxides and one chemical element inweight percent:

Percent Theoretical Analyzed 1 Not analyzed.

The differences are believed to be due to alumina pickup andvolatilization loss in the case of ZnO.

This glass-ceramic as an article was prepared by placing the glass as anarticle about 2 inches thick and at 1300-1700 C. in a preheated oven at1000 F. The oven temperature was increased to 1150 F. because of thepresence of the hotter glass article and oven was then maintained at1150 F. for 3 hrs. followed by increasing the oven temperature to 1350F. at a rate of 5 F./ minute and then maintained at 1350 F. for 50hours. The glass article then was cooled at the rate of 1 F./minuteuntil 1000 F. was reached, then 5 F./ minute until room temperature wasreached.

Copending U.S. patent application Ser. No. 362,481 filed by Robert R.Denman on Apr. 24, 1964 and now US. Pat. 3,428, 513, and entitledCeramics and Method with common assignee describes a process ofimproving the modulus of rupture of certain compositions of glassceramicby an ion-exchange process in which lithium ions in a surface layer ofthe glass-ceramic article are replaced by larger alkali metal ions,specifically sodium or potassium ions. The glass-ceramic is in the formin which the crystalline phase is primarily beta-spodumene crystals andbeta-spodumene-like crystals. To date none of the ionexchange materialsused has provided a similar result With a strength increase for aglass-ceramic of such certain compositions, that are specified in saidapplication Ser. No. 464,147 and in the next paragraph in which thecrystalline phase is primarily beta-eucryptite or beta-eucrptitelikecrystals.

That Denman patent application is also incorporated by reference. Thethermally crystallizable glass composition that forms the glass-ceramicused has the following weight percentage limits based on the totalcomposition:

SiO 68-72 A1 0 16-18 Li O 3-4 MgO 3-5 ZrO 1-2 Tio 1.2-2.4 r 0 0.8-2

In addition, small amounts of residual arsenic and antimony oxides areoften present in the composition, since arsenic or antimony compoundsare often used as fining or oxidizing agents. In actual practice,arsenic, expressed as AS 03, is usually present in amounts not more than0.3 weight percent, and antimony, expressed as Sb O is seldom present inamounts over 1 weight percent. Sodium oxide is often present in theglass to a certain degree from the raw materials, usually in amounts notover 1.5 or 2 weight percent. Further, when AS203 is used as a finingagent, it is commonly added together with a little NaO a we1lknownpractice. Another additive sometimes employed is F, usually in amountsnot exceeding 0.3 weight percent. It is added as a salt in the usualmanner and seems to aid the crystallization process somewhat when it isemployed. Thus, it seems to accelerate the rate of crystallization,sometimes to such an extent that harmful exothermic effects result;hence, it is usually undesirable to have any more than 0.25 to 0.3fluorine present, expressed as weight percent F, in the final glasscomposition.

The glass is formed, e.g., by melting a batch of suit able ingredientsin a gas-fired furnace at a temperature of about 2900 F. and afterfining is cooled to a suitable temperature for flowing, casting or anyother feeding step to form glass articles which are then heat treated,first, at a low temperature to form many nuclei or crystallites, andthereafter at a higher temperature to complete crystallization to thedesired degree. The final maximum crystallization temperature is about1800 F. to 2100 F. and the average coefiicient of linear expansion(0-300" C.) is less than 20 10""/ C. and usually about 15 10' C. In oneexample a glass-ceramic article having, and made from a thermallycrystallizable glass having, the following analyzed composition wasimmersed in a molten bath of sodium nitrate at 750 F. for one-half hourfor some articles and for 3 hours for others, following by cooling,water washing and drying:

Ingredient: Weight percent SiO 70.4 A1 0 16.8 MgO 4 Li O 3.5 ZrO 1.3 TiO1.8 113205 0 0'; Na O 0.5 As O 0.15

The original glass had an annealing point of about 1220 F. Theion-exchanged glass-ceramic articles had very high unabraded and highabraded values of modulus of rupture.

The preceding paragraphs refer to ion exchange for the purpose ofimproving the strength of a specific type of glass-ceramic. Before thatinvention the prior art had disclosed the technique of changing theproperties of glass articles by ion exchanging one alkali metal foranother in the surface layer of the glass article. This ion-exchangeprocess can be one of two types of substitution. In the one embodimentthe replacing alkali metal ion has a larger ionic diameter or radiusthan the alkali metal ion being replaced. In the second embodiment thereplacing alkali metal ion has a smaller ionic diameter than that of thealkali metal ion being replaced.

H. G. Fischer and A. W. LaDue disclose and claim in their copending U.S.patent application Ser. No. 504,159, filed on Oct. 23, 1965, now U.S.Pat. 3,481,726 and entitled Process and Product with common assignee, amethod in which ion exchange of one alkali metal for another isaccomplished by using a liquid medium containing an alkali metal salt ofan organic acid.

E. F. Grubb and A. W. LaDue in their copending U.S. patent applicationSer. No. 529,215, filed on Feb. 23, 1966, now U.S. Pat. 3,498,773 andentitled Process and Product with common assignee, disclose and claimanother ion-exchange method in which the alkali metal ion, that is tosubstitute for another alkali metal ion in the surface layer of theglass article, is used as a compound that is not molten when in contactwith the glass at the elevated temperature used for the ion exchange.

In view of the methods of said Fischer et al. and Grubb et al., herebyincorporated by reference, it will be apparent that there have now beendeveloped several different techniques for ion exchange.

The ion-exchange process has been used to treat glass, that is notthermally crystallizable, to convert a surface portion of the glassarticle to a composition that at the temperature used for the ionexchange, if sufiiciently high, will crystallize to form a glass-ceramicin which the crystals are referred to as beta-spodumene crystals. If theentire article were of this surface layer composition, it is reasonableto expect for some specific compositions that only the surface wouldcrystallize. The product of this process is referred to as a surfacecrystallized glass article, as distinguished from the conventionalglass-ceramic which is commonly referred to as bulk crystallizedproduct. As a result of this process in which sodium ions are replacedby lithium ions of a molten lithium salt bath in which the glass articleis immersed at the elevated temperature, the compositional change issuch that surface crystallization occurs while the main body or interiorportion remains unchanged in composition and thus remains as glass. Thisprocess is diclosed in U.S. Pat. No. 2,779,136.

Copending U.S. patent application Ser. No. 371,089, filed on May 28,1964, now U.S. Pat. 3,522,828 by William E. Smith and entitled Glass,Ceramics and Method with common assignee, discloses and claims a type ofthermally crystallizable glass that, for example, as a glass has anexpansion coefficient of about 90 10- C. but as a glass-ceramic has anexpansion coeffcient between 100 10 C. and 120 10"/ C. For theglassceramic the actual value of the coefiicient is determined by thetemperature and time of heat treatment for the controlledcrystallization. The crystalline phase of that glass-ceramic isnepheline. That application is hereby incorporated by reference.

Another type of composition of thermally crystallizable glass and theglass-ceramic made from it are disclosed in British specification No.869,328. The ingredients include silica, alumina and soda and thus theglassceramic has nepheline as its primary crystalline phase.

William E. Smith in another copending U.S. patent application, which isapplication Ser. No. 532,058, filed on Mar. 7, 1966, now U.S. Pat.3,486,963 and entitled Process and Product with common assignee,discloses and claims a process for treating an article of a glassceramic to convert at least an area of its surface layer to anoncrystalline glass under conditions to maintain the main body portionof the article as a glass-ceramic. That invention requires that theglass-ceramic has a coefiicient of linear expansion that is at least andis a maximum of about 200% greater than that of the noncrystalline glassof the same overall chemical composition, which, of course, is the glassformed as a surface layer by the process. That application is herebyincorporated by reference.

Y 8 OBJECTS OF THE INVENTION It is an object of the present invention toprovide an article with a main body portion that is a glass-ceramic andat least one surface layer that is either a glassceramic or a glass, inwhich the surface layer (1) has a lower coefiicient of linear expansionthan that of the main body portion, (2) has a compressive stress whereasthe main body portion has a tensile stress, and (3) differs (a) byoverall chemical composition from that of the main body portion, (b) bygreater crystal concentration or by the predominant crystal of thecrystalline phase of the glass-ceramic from the crystal concentration orfrom the predominant crystal, respectively, in the crystalline phase ofthe glass-ceramic of the main body portion, or (c) by its glass matrixhaving a different chemical composition, e.g., higher lithia content,than that of the glass matrix of the main body portion.

It is another object of this invention to provide processes for themanufacture of such article.

It is still a further object of the invention to provide a process ofmanufacturing a glass-ceramic article by in situ crystallization of aglass article by a process that minimizes change in shape of the articleduring the controlled heat treatment to convert the glass to aglassceramic, and especially to use this process as part of an overallprocess to make an article that is of the type recited above as thefirst object of the present invention.

These and other objects of this invention will be apparent to oneskilled in this art from a description of various embodiments of theinvention that follow.

DESCRIPTION OF THE INVENTION The following examples are presented forthe purpose of illustrating various embodiments of the process of thepresent invention and, of course, the products obtained by theseembodiments of the process are illustrative of the product of thisinvention. The first example is a description of one embodiment, alongwith details of illustrative conditions used in carrying out the processand an evaluation of the product obtained from the standpoint of itsflexural strength, i.e., modulus of rupture, without abrasion of theproduct and after abrasion. For comparison there are presented theunabraded and abraded flexural strengths of untreated glass-ceramicobtained by the in situ crystallization but without the treatment forion exchange. The temperature used for the ion exchange will result inan in situ crystallization of a suitable glass for the production ofbeta eucryptite crystals as a crystalline phase.

The flexural strength or modulus of rupture of a glass or aglass-ceramic is determined in the following manner Glass cane isobtained by pulling it from molten glass. The glass cane is cut into5-inch long sample rods that have a diameter of about 7 inch.

When the flexural strength is to be determined for glassceramic obtainedfrom such glass, as is the present case, the glass cane is converted toa glass-ceramic by a heat treatment. First the cane is heated to andmaintained at a suitable temperature for the formation in the glass ofcrystals of a nucleant, such as titania or titania and zirconia,followed by a predetermined pattern of heat treatment at highertemperatures to obtain a crystalline phase from the main components ofthe glass. The balance of the glass ingredients remain as a glassmatrix. The resultant glass-ceramic sample rods are then tested forflexural strength after or without abrasion. In the present case theabrasion comprised manually rubbing the sample rods of glass-ceramicwith No. 320 emery paper.

The flexural strengths of the sample rods are determined using aTinius-Olson testing machine. This machine applies a measured loadthrough a single knife edge to the center of the sample rod supported ontwo knife edges that are four inches apart (3-point loading).

The load is applied at a constant rate of 24 lbs. per min. until failureoccurs with a marker indicating the highest load applied to the point offailure. A dial micrometer calibrated in inches and equipped with a barcontact instead of a point contact wasused to measure the maximum andminimum diameters at the center of the sample to an accuracy of 0.0005inch. Since few sample rods are perfectly round, the load is appliednormal to the maximum diameter and the standard formula for anelliptical cross-section is used in calculating the modulus of rupture(MR) as follows:

(10.185) X Load 1 X 2 The modulus of rupture in this formula gives theflexural strength in pounds per square inch of cross-sectional area atfailure. The data for flexural strengths are based on the average of thevalues obtained for a number of sample rods.

EXAMPLE I A molten glass was made with batch materials to provide thefollowing composition:

This glass was made in a pot in a conventional manner that is well knownin the art. This glass had a liquidus of greater than 2760 F. Glass canewas drawn from the molten glass. The silica, alumina and magnesiacontents were within the broad and preferred ranges of glasses that areutilized in the process disclosed and claimed in US. Pat. No. 3,117,881.The glass contained, as nucleants, titania and a larger amount ofzircona. These are utilized in various compositions of that patent. Thetin oxide would provide the function in the glass manufacture that isdescribed in that patent. The 1.5% by weight of soda in the glass waswithin the broad and preferred ranges of modifying agents permitted inthe thermally crystallizable glass of that patent. This specific glasshad an average coefficient of linear expansion (-300" C.) of 33.7 C.

The glass can was cut into sample rods. Most of the sample rods wereheat treated as follows to convert them to a glass-ceramic. These rodswere heated to and maintained at 1450 F. (788 C.) for 1% hours, heatedto and maintained at 1750 F. (954 C.) for 1 /2 hours, heated to andmaintained at 2000 F. (1093 C.) for 1 /2 hours, and then cooledgradually to room temperature. By this treatment the glass of the rodswas converted to a glass-ceramic. Cordierite crystals are thepredominant crystals in the crystalline phase. This glass-ceramic had anaverage coefficient of linear expansion (0-300 C.) of 59.0 l0- C. whichis greater than that of the glass.

Sample rods of this glass-ceramic after preheating for one-quarter hourat 1450 F. while suspended in a tubular furnace were moved laterally bymoving the tubular furnace directly above another furnace containingsalt in a crucible liner in a metal container. The latter furnace wasmaintained at a temperature of 1450 or 1500 F., at which temperaturesalt was molten. The molten salt bath contained, on a weight basis, 75%lithium sulfate, 24% potassium sulfate and 1% sodium hydrogen sulfate.The rods were lowered for immersion in the molten salt for specificperiods of time, raised from the bath, cooled gradually in air, waterwashed and then dried.

Most of the ion-exchanged treated sample rods of glass-ceramic wereabraded, as described above. Some were tested for flexural strengthwithout any abrasion.

Time of Flexure strength, p.s.i.

immer- Temperature of salt bath, F. sion, hrs. Unabraded Abraded As seenin some of the copending patent applications mentioned above, atemperature of 1450 and 1500 F. will provide an in situ crystallizationto form a crystalline phase that contains predominantly beta-eucryptite.Thus the temperature of the molten salt treatment was sufficientlyelevated to provide in that exterior part at least of the surface layeran in situ crystallization of part of the glass matrix of the initialglass-ceramic changed in composition by replacement of sodium ions bylithium ions. The coefficient of linear expansion of 59 10-' C. of theinitial glass-ceramic would certainly be greater than that in thesurface layer after substitution of lithium for sodium in the glassmatrix and especially after conversion of part of the changed glassmatrix in at least the exterior part of the surface layer tobeta-eucryptite in view of its very low expansion coefficient ascompared with cordierite. Thus the process provides in at least theexterior part of the surface layer by ion exchange, a reduction in theexpansion coeflicient. This can account for the improved flexuralstrength after the process of ion exchange under conditions that providealso in situ crystallization. This compressive stress surface layer alsoprovides a retention during a specific abrasion of flexural strengthgreater than that of the unabraded initial glassceramic.

For comparison, sample rods of the initial glass were similarlypreheated for one hour at 1400 F. or 1450 F., were immersed for one-halfhour in that molten salt at the temperature of preheating .and thentreated further. In the case of the glass rods that were immersed at1400 F., they were slowly cooled in air at the conclusion of the saltimmersion. An examination of these treated glass rods indicated that nocompressive stress surface layer had been formed. In the case of theglass rods immersed in the molten solt at 1450" F., they were posttreated after the immersion in the molten salt by heating for one hourat 1720 F. (938 C.) and then for another 1 /2 hours at 1920" F. (1049C.) which resulted in rods that had wrinkled surfaces. It was concludedthat these treatments of glass rods resulted in relatively weak rods.These results in comparison with those using the ion-exchange salttreatment of the glass-ceramic rods show that, with this type of overallcomposition, the improvement in strength is obtainable only when anarticle is treated with molten salt at such temperature when thiscomposition is al glass-ceramic rather than a thermally crystallizable gass.

Pure stoichiometric cordierite is a magnesium aluminosilicatecrystalline material with the formula The glass composition of thisexample by the in situ crystallization produces a cordierite crystallinephase and a glass matrix in which the magnesia content and the sodacontent are substantially less than and substantially greater,respectively, than that of the initial glass. The replacement of sodiumions by lithium ions in the surface layer of such glass matrix providesa lithia content for the glass matrix such that an in situcrystallization of beta-eucryptite occurs at the ion-exchange treatmentif sufliciently high or if not by later heat treatment. This lowersadditionally the expansion coeflicient of the surface layer. Theinterior portion of the surface layer may not 1 1 have suflicient lithiacontent to provide crystallization, but it will lower somewhat theexpansion coefficient of that portion of the surface layer.

If a temperature sufficiently lower than that used in Example I for theion exchange is used, the time of treatment would be increased for acomparable degree of ion exchange. Such lower temperature can besufficiently low that in situ crystallization will not occur. Then theglassceramic after such ion exchange with suitable lithiumcontainingmedium is followed by a heat treatment of the rods at an elevatedtemperature sufficient to provide the in situ crystallization, therebyattaining generally the same result as described above for theion-exchange treatment at an in situ crystallization temperature.

In view of the foregoing presentation of prior art and Example I for oneembodiment of the present invention, it is not necessary to presentother embodiments of the invention in the same amount of detail. Theseembodiments can be utilized by one of ordinary skill in this art in viewof this entire disclosure and the prior art.

The embodiment that is illustrated by Example I is the process ofconverting an article of thermally crystallizable glass composition to aglass-ceramic followed by replacement of an alkali metal ion of thesurface layer of the glass matrix by an alkali metal ion having asmaller ionic radius at a temperature that provides in situcrystallization or at a lower temperature followed by heating at atemperature for such crystallization in at least the exterior portion ofthe surface layer wherein the formed crystals are different than thosein the crystalline phase of the initial glass-ceramic and have a lowercoeificient of expansion. Of course, in this example the surface layerwill contain, in addition to the newly formed crystals, the crystals ofthe main body portion of the glass-ceramic rods but these newly formedcrystals are not necessarily separate from the initial crystallinephase. They may be formed on the surfaces of the initial crystallinematerial and, of course, formation of the new crystalline material asseparate crystals is not a necessary part of the present invention.Furthermore in certain cases ion exchange without concomitant orsubsequent in situ crystallization is one embodiment of the presentinvention, whether the surface layer is glass or glass-ceramic. Ofcourse, the main body portion is glass-ceramic.

EXAMPLE II An article of thermally crystallizable glass composition ofthe type, that by in situ crystallization forms a glassceramic with acrystalline phase that is predominantly nepheline, is converted to anarticle of glass-ceramic by suitable heat treatment, as describedearlier with reference to British Pat. No. 869,328 and Smith patentapplications Ser. Nos. 371,089 and 532,058.

In one embodiment the heat treatment at a specific final temperature isfor a period of time less than onehalf of that which will produce themaximum amount of crystalline phase so that the glass matrix of theglassceramic will contain a substantial amount of soda. In thisembodiment the glass-ceramic article is treated with alithium-containing ion-exchange material, such as the molten mixture ofsalts including lithium sulfate mentioned above, at an elevatedtemperature that is at least about 200 C. (about 400 F.), and preferablyat least about 350 C. (about 660 F.), for a period of time to replacesodium ions by lithium ions in the glass matrix in a surface layer ofthe glass-ceramic article. This ion exchange converts the glass matrixin the surface layer to a glass composition having a lower expansioncoeflicient than the initial glass matrix and thus lower than that ofthe glass matrix in the main body portion of the glass-ceramic article,so that the surface layer has a compressive stress and the main bodyportion has a tensile stress. When the ion-exchange treatment isconducted at an elevated temperature that is below the strain point ofthe glass matrix, it is necessary to heat the article after the ionexchange to a higher temperature sufiicient to relieve tensile stress inthe surface layer created by the smaller ions replacing the larger ions.The compressive stress in the surface layer is created because of thedifference in expansion coefficient between that layer and the main bodyportion. It is preferred that the ion exchange be conducted at atemperature above the strain point of the glass matrix of the initialglass-ceramic article. A temperature between 500 C. (about 950 F.) and850 C. (about 1560 F.) is preferred. A temperature of at least 700 C.(about 1300 F.) is especially preferred. When the time and temperatureof the ion exchange are sufficiently long and high, respectively, andwith suitable choice of initial glass composition and of extent ofcrystallization prior to the ion exchange, there is a further in situcrystallization in the surface layer. The new crystals arebeta-eucryptite. The surface layer already contains a crystalline phasewhich, of course, is predominantly nepheline. This additionalcrystallization in a surface layer results in a different glassceramiccomposition than that of the main body portion. Beta-eucryptite has alower expansion coefficient than nepheline. Thus the glass-ceramic ofthe surface layer has a lower expansion coeflicient than the main bodyportion. Also this glass-ceramic of the surface layer has a lowerexpansion coefiicient than that of the surface layer in which only ionexchange occurs.

In another embodiment using this glass-ceramic in which nepheline is thepredominant crystal in the crystalline phase, the surface layer of theglass-ceramic article is heat treated, such as by flame treating, inaccordance with the process disclosed in Smith patent application Ser.No. 532,058, and ion-exchange treated to replace sodium ions in thesurface layer of thermally crystallizable glass by ions in the mannermentioned above. The glass of the surface layer before the ion-exchangetreatment has a lower expansion coefficient than the glass-ceramic mainbody portion. This difference is further enhanced by this ion exchange.By judicious choice of initial glass com position and degree of ionexchange, part of the glass surface layer can be converted to crystalsof beta-eucryptite with heat treatment at temperatures describedearlier, because such compositions contain substantial amounts ofnucleant and substantial contents of the three ingredients that formbeta-eucryptite.

The process of the Smith application Ser. No. 532,058 excludes the useof a glass-ceramic that has a lower coeflicient of linear expansion thanthe thermally crystallizable glass from which it is obtained. Theconversion of a surface layer of an article of such glass-ceramic to theglass followed by cooling the glass layer results in spalling orbreaking away of the surface layer because a tensile stress is createdin the surface layer by this difference in expansion coeflicient.

If there is an ion exchange of a larger ion for a smaller ion afterforming the glass layer as in one embodiment of that Smith process, thecooling is only to a temperature below the annealing point and usuallybelow the strain point of the glass at which the exchange takes place toprovide a compressive stress. However, cooling to such temperature alsocreates this undesirable tensile stress.

The second embodiment of the present invention avoids such spalling,because the ion-exchange process of such glass surface layer can becarried out at a temperature above the annealing point. A subsequentcooling of the article to room temperature without damage by spalling ispossible, because the surface layer now is a glass that has a lowerexpansion coefiicient than that of the glassceramic main body portion.When the process of this invention converts the ion-exchanged glasssurface layer to a glass-ceramic layer during or after the ion exchange,the article is ultimately cooled to room temperature. There is nospalling damage, because the glass-ceramic of the surface layer has asubstantially lower expansion coeflicient than that of the glass-ceramicof the main body portion and thus a compressive stress has been createdin the surface layer.

The foregoing advantage of this embodiment, as compared with thelimitation of the process is because an alkali metal ion is replaced byanother alkali metal ion having a smaller ionic radius. This replacementis performed at or is followed by heating the article to a temperatureabove the strain point, even above the annealing point.

EXAMPLE III This example uses an article of a thermally crystallizableglass composition such as disclosed in Table II of U.S. Pat. No.2,920,971 or U.S. patent applications Ser. Nos. 352,958, 362,481 and464,147.

In one embodiment, thermally crystallizable glass of the article isconverted to a glass-ceramic in which the crystalline phase ispredominantly beta-spodumene and beta-spodumene-like crystals so thatthe article of glassceramic formed has an expansion coefiicient betweenl 10-"/ C. to 20 10 C., such as 1S 10-' C. The surface layer of thisglass-ceramic article is heated to convert it, without converting themain body portion, back to the thermally crystallizable glass. Thisglass is converted by heat treatment, under conventional conditions ofthe type described earlier in connection with the prior art, to convertit to a glass-ceramic in which the crystalline phase is predominantlybeta-eucryptite. The resultant article has a surface layer of adifferent glassceramic with a lower coefiicient of linear expansion thanthat of the glass-ceramic of the main body portion. Based upon thethermal treatments received by the glass of the surface layer and theglass of the main body portion, it is expected that the surface layerhas an expansion coeflicient, for example, of l0 10-"/ C. while the mainbody portion has an expansion coeflicient of x 10-"/ C. In this case,there is no difference in overall chemical composition between thesurface layer and the main body portion. The difference is in the natureof the crystals of the crystalline phase of the two glass-ceramics thatconstitute the surface layer and the main body portion.

In the foregoing first embodiment of this example, the initial thermallycrystallizable glass has a higher expansion coefiicient than that of theglass-ceramic of that composition and containing beta-spodumene. Theconversion of the surface layer of the article from this glass-ceramicto the thermally crystallizable glass results initially in a producthaving a surface layer with a higher expansion coefficient than that ofthe main body portion. In this embodiment, it is desirable, and when thedifference is too great, is necessary that the article is maintainedabove the strain point and preferably above the annealing point betweenthe heat treatment that forms the glass surface layer and theheat-treatment that converts the glass of the surface layer to aglass-ceramic in which beta-eucryptite is the predominant crystallinematerial. Accordingly, the entire article is preferably heated to asubstantial elevated temperature, preferably above the annealing pointof the thermally crystallizable glass, before the heat treatment, suchas flame treatment, of a surface layer of the glassceramic to form theglass surface layer and the entire body is maintained at suchtemperature until in situ crystallization occurs in the surface layer.Of course, this temperature is below the temperature at whichcrystallization occurs to form beta-spodumene. This is the preferredembodiment of the present invention.

Under certain conditions including the maximum temperature reached bythe surface layer and dependent upon the titania content of the initialthermally crystallizable glass, titania may be redissolved when thesurface layer is heated to form a surface layer of glass. In such event,the temperature of the entire article is maintained such that thetemperature of the newly formed glass surface layer is lowered to thenucleation temperature. The tem- 14 perature of the article is thenraised to caused the in situ crystallization for beta-eucryptite crystalformation by a programmed heat treatment as described in the U.S. patentand patent applications mentioned above.

In a second embodiment of this example, the article is made of thermallycrystallizable glass that contains only a minimum content of lithia anda maximum amount of soda but an adequate content of nucleant to providebulk crystallization. The article is entirely converted to the glassceramic by controlled heat treatment. The article is ion exchanged inits surface layer to replace sodium ions with lithium ions. Thetemperature during the ion exchange is preferably at the high elevatedtemperatures mentioned above. At lower exchange temperatures tensilestress is created, but the later in situ crystallization temperaturerelieves such stress and creates compressive stress. However, While atthe lower temperature damage to the article can occur in certain cases.The time and temperature for ion exchange can provide for an in situcrystallization of beta-eucryptite in the surface layer during at leastthe final period of the ion-exchange treatment. This glass-ceramicobtained from the ion-exchanged surface layer has a lower expansioncoefiicient than that of the main body portion. This aspect of thisinvention is thus (1) the difference in expansion coefficients of theglass matrix of the surface layer and the main body portion, (2)additional crystal formation with added lithia to form beta-eucryptiteeither during the ion exchange or later, (3) reduced soda content in theglass matrix of the surface layer and (4) reduced molar content of allalkali metal in this glass matrix that results in a lower eXpanioncoefficient than the initial glass matrix.

The second embodiment is modified by use of a final temperaturesufficiently high to convert lithium values introduced by the ionexchange to beta-spodumene. This changes the composition of the glassmatrix to a composition having a lower expansion coeflicient.Furthermore, the glass-ceramic of the surface layer has a lowercoefiicient of expansion in this case because the concentration ofbeta-spodumene in the surface layer is greater than that in the mainbody portion. In this modification the surface layer, that is ionexchanged, is the initial glass-ceramic or the glass obtained from it.

In another embodiment of this example, the thermally crystallizableglass composition of the article may contain a small soda concentrationor content. It is preferred that the soda content be very low or thatsoda be absent. The article of this glass is ion exchanged to replacepart of the lithium ion content with sodium ions using one of theWell-known ion-exchange processes. The temperature is not required to bebelow the strain point for a long time of treatment or at a maximumtemperature of about F. above the annealing point for a short time,because this ion exchange is not performed to create a compressivestress surface layer. Thus the preferred temperature is substantiallyabove the annealing point but below the softening point of the glass.

During the ion exchange of this embodiment the temperature may be suchthat nucleation may occur. The degree of nucleation may be insufficient.Thus the article after the ion exchange is subjected to heat treatmentfor nucleation. The glass article, now ion exchanged in a surface layerand nucleated throughout, is heat treated as indicated in the patent andapplications mentioned above to convert the main body portion to aglassceramic.

The extent of ion exchange is controlled for certain articles so thatthe composition of the surface layer is such that it will also formglass-ceramic, but at a slower rate than the crystallization of the mainbody portion. Th1s is desirable to avoid flow of the glass surface layerat the high temperatures required for this in situ crystallization ofthe main body portion. Obviously, this is not a necessary limitationwhen only one surface of the article is ion exchanged and that surfaceis fiat, because it can be maintained horizontal and above the main bodyportion to prevent gravity flow with peripheral guards, if necessary,during the heating for in situ crystallization.

After the article has its main body portion converted to theglass-ceramic, the article is then ion exchanged to replace sodium ionswith lithium ions in the entire surface layer, whether the surface layeris glass or glassceramic with its content, i.e., concentration, ofbetaspodumene being less than that in the main body portion. Then thearticle is thermally treated under the proper program of heat treatment,as disclosed in patent mentioned above, to convert the introducedlithium and some ingredients including lithium already present tocrystals of beta-eucryptite. In view of the replacement of soda contentin the surface layer, this heat treatment by in situ crystallization tobeta-eucryptite utilizes lithia content that did not form beta-spodumenewithin the time of the first in situ crystallization. Thus the surfacelayer is a glassceramic and now has a lower expansion coefficient thanthat of the main body portion.

EXAMPLE IV The glass of the entire initial article is a thermallycrystallizable glass composition of the type of Example III. This glasscontains an amount of soda within the top portion of range of sodacontent that does not prevent in situ crystallization to formglass-ceramic that will contain either beta-eucryptite or beta-spodumeneor both. The glass article is treated by ion exchange to replace sodiumin its surface layer by lithium in accordance with any of the processesof ion exchange already known for glass, such as are described above.This results in a surface layer of thermally crystallizable glasscomposition that differs from that of the main body portion. The surfacelayer has a higher lithia content and a lower soda content than that ofthe main body portion. 'In this embodiment, the nucleating agent oragents are present at concentrations suflicient for bulk crystallizationonly at a slow rate, e.g., a rate less than one-fourth of the maximumrate obtainable with maximum permissible content of nucleant.

This article of glass with a surface layer of ion-exchanged glass isheat treated to a temperature that will provide a surfacecrystallization, i.e., in an in situ crystallization in the surfacelayer, to form beta-eucryptite. This surface crystallization will occurbefore in situ crystallization of the main body portion, because thesurface crys tallization is not dependent upon the presence of specialnucleants in the glass.

It is known in the prior art, as illustrated by the teaching of US. Pat.No. 2,779,136, that surface crystallization can occur at temperatures aslow as between 600 C. and 750 C. (between 1112 C. and 1382 F.).Thesetemperatures are below temperatures for bulk in situ crystallization.The advantage of this surface crystallization is that the subsequentprogram of heat treatment, including a nucleation heat treatment, iscarried out on an article that has a surface layer of glass-ceramicrather than glass. This surface layer of glass-ceramic avoids orminimizes change in shape due to sagging or flow during the bulkcrystallization temperature treatment.

This article, with its surface layer of glass-ceramic due to surfacecrystallization, is subjected to a program of heat treatment to convertthe entire article to a glassceramic that is predominantlybeta-eucryptite or betaeucryptite-like crystals or a mixture of theseand betaspodumene and beta-spodumene-like crystals or predominantlybeta-spodumene and beta-spodumene-like crystals. The process of thisexample uses a separate facet of the invention that does not require ionexchange and other steps subsequent to the bulk crystallizationtreatment.

EXAMPLE V The entire article obtained by Example IV is converted by heattreatment of prolonged duration to a glassceramic in whichbeta-spodumene and beta-spodumene- Example lV describes an aspect of theinvention in which there is an ion exchange of an article of a thermallycrystallizable glass to convert a surface layer to a composition that,by heat treating within a temperature range, will surface crystallizewithout bulk crystallization. The composition of the original glass willbulk crystallize but at a slow rate due to its specific composition. Ionexchange changes this composition in the surface layer so the latterwill easily and rapidly surface crystallize. These thermallycrystallizable glasses form glass-ceramics in which alternativelybeta-eucryptite and beta-spodumene crystals or mixtures of both arepresent.

This advantage of surface crystallization of a glass prior to bulkcrystallization can be utilized with other types of thermallycrystallizable glasses, e.g., those that form a glass-ceramic in whichnepheline or cordierite are the predominant crystals. The glass is ionexchanged to replace sufficient sodium in the surface layer by lithium.During the ion-exchange treatment, if its temperature is suflicientlyhigh, or after it treatment at an elevated and suitable temperatureproduces surface crystallization. This article is heat treated, withless change of its shape, to a glass-ceramic for the main body portionthat is either a nepheline type, a cordierite type or some other typeother than beta-spodumene or beta-eucryptite. The glass-ceramic of thesurface is converted to a glass-ceramic in which the crystals arebeta-eucryptite or beta-spodumene or both, dependent upon the finaltemperature required for in situ crystallization of the main bodyportion. That. temperature depends on the type of initial glasscomposition.

The initial glass can be modified by incorporating a limited amount oflithia. This initial lithia content along wth the ltihia contentprovided by the ion exchange gives a surface layer composition that isconvertible to the glass-ceramic by surface crystallization as describedabove.

After the bulk crystallization, this article has a compressive stresssurface layer due to the difference in expansion coefficients betweenthe main body portion and the layer that are two differentglass-ceramics.

EXAMPLE VII The article obtained in Example VI is selectively heated toconvert the surface layer to glass, while the main body remains asglass-ceramic followed by heat-treatment to glass-ceramic in whichbeta-eucryptite is the predominant crystal.

EXAMPLE VIII A glass article having the composition of Example VI ision-exchanged, surface crystallized and then bulk crystallized. Thesurface layer is then ion exchanged to substitute sodium for lithium inthe glass matrix that was not utilized in the surface crystallization toform, by further heat treatment, additional crystals comparable to thosein the main body portion. Thus the second ion exchange replaces lithiumby sodium.

EXAMPLE IX Thhe surface layer of the article of Example VI after thebulk crystallization is selectively heat treated, such as by flametreating, to form a glass, while the main body remains as glass-ceramic.This glass is ion exchanged to replace lithium with sodium followed byin situ crystallization at the surface layer so that the surface layerand the main body portion now have the same over all composition. Inthis alternative, the surface layer is used only 17 for surfacecrystallization to minimize shape change during subsequent bulkcrystallization. It is not used for the objective of making a finalarticle with a surface layer having a compressive stress due to a lowerexpansion efiicient for the surface layer as compared with a differentoverall composition than that of the main body portion.

EXAMPLE X An article of a glass containing on a Weight basis thefollowing ingredients: 62.9% SiO 14% A1 0 6% MgO; CaO 1.7% Li 0; 4.3%TiO 3.1% Na O and 3% B 0 is made. This glass has an expansioncoefficient between 25 C .and 300 C. of 45 X- C. This glass is Example 1of Table '1 of US. patent application Ser. No. 410,016 entitled Glass,Ceramic and Methods filed by Richard W. Petticrew on Nov. 9, 1964, nowUS. Patent 3,540,893 with common assignee, and hereby incorporated byreference.

By heat treatment for two hours at 1300 F., two hours at 1450 F. and onehour at 1550 F., it is converted to a glass-ceramic with an expansioncoefiicient, for the same temperature range, of 43 10 C. and a modulusof rupture of 66,000 p.s.i. The high strength is attributed by Petticrewpartially to a formation of lithium-containing crystals in a largeramount in the surface layer than obtained in the main body portion ofthe article. In the main body portion the crystalline phase of theglassceramic is predominantly cordierite.

This article of thermally crystallizable glass is treated, in accordancewith the present invention, by an ion-exchanging material, such asdescribed earlier, to replace at least part of sodium ions by lithiumions in the surface layer of the article, thereby increasing the lithiacontent of the surface layer. In the present process, the article isthen heat treated as described by the Petticrew patent application forbulk crystallization. Because of the ion exchange a higher concentrationof lithium-containing crystals, e.g., beta-eucryptite is formed in thesurface layer to provide a lower expansion coeflicient for the surfacelayer than that obtainable by the heat treatment without the ionexchange.

In another embodiment, the glass-ceramic article obtained in Example 1of Petticrews patent application is heated to convert a surface layer tothermally crystallizable glass while maintaining the main body portionas glass-ceramic. This glass surface layer is ion exchanged to replacesodium with lithuim. The heat treatment of the ion-exchanged glass layerto glass-ceramic containing beta-eucryptite or beta-spodumene is thenperformed, as described in earlier examples.

The foregoing description of compositions has mentioned variousingredients. These constitute at least 90% and preferably at least 95%,by weight of the compositions.

The depth of the surface layer of the article of the invention can bevaried widely, e.g., from 10 microns to 200 microns or more. When thissurface layer is formed by ion exchange, the depth obtained by thisexchange is desirably less when a later ion exchange is used as part ofthe overall process because the second ion exchange should replace theion first substituted with an ion of the type first replaced. To do thiseffectively a shallow layer is desired in the first exchange.

The difference in average coeflicient of linear expansion between thatof the surface layer and that of the main body can be varied widely. Theminimum numerical value for the difference is dependent upon thecoefficient of the main body portion. If it is 10 10-' C., thecoefficient surface layer is desirably no higher than about 5 l0-"/ C.,and preferably no higher than 0x10 C. If it is 10-' C., that of thelayer is preferably no higher than 10x 10 C. If the coefficient of themain body portion is about 60 10-' C., the layers coefficient ispreferably a maximum of 30X 10-"/ C. which is easily accomplished whenthe layer is a glass-ceramic containing beta-eucryptite, beta-spodumeneor both as the predominant crystal. When the coefficient of the mainbody portion is about 110 l0 C., that of the layer is preferably amaximum of about 10- C., and preferably at least in an outer part of thesurface layer of below 50 l0-"/ C.

Many variations of the invention in view of this disclosure will beobvious to one of ordinary skill in the art.

The foregoing examples have been presented for the purpose ofillustration of various embodiments of the process and the product ofthe present invention which is not limited thereto, but only by theclaims that follow:

I claim:

1. An article comprising a main body portion of glassceramic wherein,B-spodumene is the predominant crystal of the crystalline phase and anintegral surface layer of material selected from the group consisting ofglassceramic and glass, said surface layer having (1) an overallcomposition substantially identical to that of the main body portionexcept that (a) the lithia content, when the layer is a glassceramic, isat least equal to and, when the layer is a glass, is greater than thelithia content in the overall composition of the main body portion,

(b) the content of oxide of an alkali metal, having an ionic radiuslarger than lithium, when the layer is a glass-ceramic, is at a maximumequal to and, when the layer is a glass, is less than the content ofthat oxide in the overall composition of the glass-ceramic main bodyportion, and

(c) when the layer is a glass-ceramic with lithia content equal to thatof the main body portion, the predominant crystal in the crystallinephase of the layer differs from that of the main body portion andprovides the lower coefficient of linear expansion of the layer, and

(2) a lower coeflicient of linear expansion than that of the main bodyportion,

said article having a greater flexural strength than an articlecontaining only the same glass-ceramic as that of the main body portion.

2. The article of claim 1 wherein the surface layer is a glass-ceramiccontaining B-eucryptite as the predominant crystal of the crystallinephase.

3. The article of claim 2 wherein the average coefficient of linearexpansion of the glass-ceramic of (1) the main body portion is at leastabout l0 10' C. and

(2) the surface layer is a maximum of 5x10- C.

4. The article of claim 3 wherein the average coefficient of linearexpansion of the glass-ceramic of the main body portion is between about10 10 C. and 20 10*' 5. The article of claim 4 wherein the averagecoefiicient of linear expansion of the glass-ceramic of (l) the mainbody portion is about 15 X 10 C. and

(2) this surface layer is a maximum of about 0X 10 6. The article ofclaim 1 wherein (l) the glass-ceramic of the surface layer contains B-spodumene as the predominant crystal of the crystalline phase,

(2) the lithia content of the glass-ceramic of the surface layer isgreater than that of the main body portion, and

(3) the content of oxide of said alkali metal is greater in the mainbody portion by a difference in mole percent approximately equal to themole percent difference in lithia content between the surface layer andthe main body portion.

7. The article of claim 6 wherein said alkali metal is sodium.

19 20 8. The article of claim 6 wherein said difierence in 3,253,9755/1966 Oleott et al. 161-1 lithia content is in the glass matrix of theglass-ceramics. 3,282,770 11/1966 Stookey et al. 1611 9. The article ofclaim 6 wherein the glass-ceramic of 3,428,513 2/1969 Denman 65-33 thesurface layer has a greater crystal concentration than 3,464,807 9/ 1969Pressau 6533 that of the main body portion. 5 3,486,963 12/ 1969 Smith6533 3,573,073 3/1971 Duke et a1 65--33 Refer n Cited 3,585,054 6/1971Karstetter 6533 UNITED STATES PATENTS DANIEL J. FRITSCH, PrimaryExaminer 3,573,020 3/1971 Karstetter 6533 3,146,114 8/1964 Kivlighn 653310 Us, 2,779,136 1/1957 HOOd et a1 6530 2 920 971 1 19 0 Smokey 5 3106.39 192

